FS#552 - Write network backend for CPnP printers

The 'selphy_go' code showed how to detect CPnP models and send JPGs over to be printed..

Using the CP900, I discovered how to send raw YMC data over CPnP. This means we can write a native CPnP CUPS+Gutenprint backend for that model.

Unfortunately we won't know if the newer selphyneo models support non-jpeg CPnP printing -- The Windows driver uses WSA/WSD and Macs appear to use AirPrint. The only way to find out is to write it first.